Control of prestressed rolling mills



Aug. 27, 1968 J. A. TRACY CONTROL OF PRESTRESSED ROLLING MILLS FiledSept. 15, 1964 llllllllll lllll Hi't JOHN ANDRE TRACY INVENTOR w I I aATTORNEY$ United States Patent 3,398,559 CONTROL OF PRESTRESSED ROLLINGMILLS John Andr Tracy, Talbot Woods, England, assignor to The LoewyEngineering Company Limited, Bournemouth, England, a company of GreatBritain Filed Sept. 15, 1964, Ser. No. 396,760 14 Claims. (Cl. 72-8)ABSTRACT OF THE DISCLOSURE A method and apparatus for controlling theoperation of a prestressed rolling mill by continuously controlling theprestressing force so that the Opening between the working rolls ismaintained at its desired distance irrespective of any changes in theroll separating force during rolling signals representing theprestressing force and the load in the spacing means are continuouslymeasured and are multiplied by a factor which includes the coefficientsof chock spring and roll spring so as to eliminate the elfect of thiselasticity upon the roll opening.

This invention relates to a method and means for controlling theoperation of a prestressed rolling mill by continuously controlling theopening between the working rolls of the mill so as to maintain thisopening at a constant value, with the aim of obtaining a rolled articleor product whose thickness remains constant throughout a rollingoperation.

As is well-known, the forces which act in a rolling mill during itsoperation cause parts of the mill to expand or to contract, includingthose parts which support the working rolls of the mill in its housings,whereby the relative position of these rolls, and thereby the size ofthe opening between them, changes. This leads to corresponding changesin the thickness of the rolled articles, which, small as these changesare, may often be critical in the case of thin, flat material, such asstrip, foil or sheet stock.

This invention is applicable in particular to a type of prestressedrolling mill in which the distance between the roll axes, and hence theopening between the rolls, is set by spacing means acting directly onthe bearing chocks of the rolls. These spacing means may be mounted inthe chocks of one roll and abut against the chocks of the other roll, sothat the distance between the rolls can be varied by displacing thespacing means relative to the chocks of the first roll while retainingtheir abutting relationship with the chocks of the other roll. Inrolling mills equipped with backing rolls for the working rolls, theaforesaid spacing means may act on the bearing chocks of the backingrolls.

The spacing means may consist of spindles mounted in the chocks of oneroll so as to be longitudinally displaceable in those chocks, whiletheir ends abut on the chocks of the other roll or on parts supported onthe last-mentioned chocks.

During a rolling operation, a force is developed which tends to separatefrom each other the rolls which by their distance from each other definethe roll opening. In prestressed rolling mills of the above type, anydiiference between the prestressing and the separating forces is takenby the spacing means and will be referred to in the following as thespacing load. In practice, the prestressing force will always be set ata value greater than the maximum separating force which, under normalconditions, can be expected during rolling operations in order to insurethat the spacing load is never reduced to zero. The spacing load causesa compressive strain to occur in the spacing means and the bearingchocks, and leads to a reduction in height both of the spacing means3,398,559 Patented Aug. 27, 1968 and the chocks, whereby the distancebetween the centres of the chocks, and thereby also the distance betweenthe axes of the rolls supported in those chocks, are diminished. Thedecrease of this distance and thus of the size of the roll opening isproportional per unit load to the elasticity or springiness of thechocks and the spacing means, which will be referred to in the followingas chock spring.

During the rolling of an article, the separating force is apt to varyfrequently, such variations being caused by changes in the thickness,hardness, temperature or other physical properties of the rolledmaterial. Hence, the spacing load, and consequently the roll opening,vary as well, and the rolled article will not be of uniform gauge.

In British Patent No. 955,164 there is described a control system forthe operation of rolling mills in which any change in the separatingforce produces automatically an equivalent change in the prestressingforce. This cancels out variations in the spacing load which wouldotherwise occur. Thus, some of the changes of the distance between thechock centres and thereby of the size of the roll opening are obviated,and the gauge of the rolled material is rendered more uniform.

However, the control method described is incomplete, insofar as it doesnot take into account those changes of the size of the roll openingwhich are the result of the deflection of the rolls and of theirbearings in the chocks, and of the flattening of the rolls under load.These changes are proportional per unit load to changes in theelasticity or springiness of the rolls which will be referred to in thefollowing as roll spring. Roll spring and chock spring are frequently ofthe same order and it is therefore desirable that roll spring should betaken into account as well.

It is relatively easy to measure directly the spacing load and therebyalso the changes in the roll opening which result from the chock springof a rolling mill. These measurements can be effected by means of loadcells placed between the chocks of one roll and the spacing meansmounted in the chocks of the other roll, as described in British PatentNo. 955,164. It is, however, difficult to measure those changes in theroll opening which are the result of roll spring, especially while themill is in operation and the rolls rotate.

It is an object of the present invention to provide a method forcontrolling the operation of a prestressed rolling mill by continuouslycontrolling the prestressing force in such a manner that the openingbetween the working rolls of the rolling mill is maintained at itspre-set value irrespective of any changes in the separating force duringrolling.

It is another object of the present invention to provide a method forcontrolling the opening between the working rolls of a rolling millduring its operation in such a manner that this opening remains duringrolling unaffected by variations both of chock spring and roll spring.

Still another and more specific object of the invention is to improvethe method and means described in British Patent No. 955,164 bycompensating additionally those changes of a roll opening which are theresult of variations of roll spring.

According to the present invention, a method for controlling theoperation of a prestressed rolling mill having spacing means for settingthe roll opening, said spacing means being mounted in the bearing chocksof one roll and acting on those of the other roll, comprises the stepsof measuring continuously during rolling the load in said spacing meansand the prestressing force, transforming these measurements intoelectric signals, establishing a predetermined ratio between two signalsrepresenting simultaneous measurements of the prestressing force and thespacing load by multiplying at least one of these signals by a factorwhich includes co-eflicients representing both chock spring and rollspring, and obtaining a signal for controlling the prestressing force bycomparing the two first-mentioned signals, after multiplication, witheach other, the prestressing force being so controlled that theinfluence of chock spring and roll spring on the roll opening iseliminated.

The signals representing the load in the spacing means and theprestressing force may be compared, after multiplication, with a thirdsignal which represents the desired thickness of the rolled material,and which acts as a reference signal; in this way, the requiredcontrolling signal for the prestressing force is obtained. The referencesignal may represent the roll opening which produces rolled material ofa given thickness.

The signals representing the load in the spacing means and thepre-stressing force may be compared with each other, aftermultiplication, in a first step to form an intermediate signal, and theintermediate signal may be compared in a second step with theafore-mentioned third or reference signal to form the controlling signalfor the prestressing force.

The intermediate signal may be obtained by subtracting from each otherthe signals representing the spacing load and the prestressing forcerespectively. Similarly, the controlling signal for the prestressingforce may be obtained by subtracting from each other the intermediateand the reference signals.

In the case of a rolling mill in which the prestressing force isproduced by hydraulic means, the controlling signal may be fed into acircuit of a servo valve which controls the fluid pressure in thehydraulic means.

The predetermined ratio which is established between signalsrepresenting simultaneous measurement of the prestressing force and thespacing load is preferably equal, or substantially equal, to

but sufliciently satisfactory results can also be obtained for ratioswhich are slightly above or below this value.

The significance of the term for the cancelling out of the influence ofboth chock spring and roll spring on the size of the roll openingbetween the working rolls is evident from the following.

Leta

R=the separating force P=the prestressing force S=the spacing load thenP=R+S (1) as stated above.

Assuming P to be constant, then an increase of the separating force by rwould result, without intervention of the automatic control according toBritish Patent No. 955,164, in

i.e., the load S would be reduced by r, and consequently the compressionof the chocks and spacing reduced by k r which would increase thedistance between the centres of the chocks and thereby the size of theroll opening by the same amount. This increase is cancelled out by thecontrol method and means described in British Patent No. 955,164, bywhich the prestressing force P is automatically increased by r upon anyincrease of the separating force R by r. Equation 2 becomes:

Equilibrium between the forces is established again, and the spacingload S resumes its initial value. Therefore, the distance between thechock centers and hence the roll axes is also the same as before. Thus,the effect of an increase of the separating force on the size of theroll opening is cancelled out, but only in so far as it is the result ofchock spring. (Any decrease of the separating force would be dealt withby the control means in a similar way.)

However, any changes of the size of the roll opening which are theresult of roll spring have so far not been taken into account. For anincrease of the separating force by r, the roll opening is increased asa result of roll spring by'k r. Hence, the increase of the roll openingresulting from both chock spring and roll spring is k r+k r, or rk (1+m)wherein as stated before. This increase in the roll opening can becancelled out by reducing the distance between the chock centers by k(1;+m). This reduction is brought about by increasing the prestressingforce by r(l+m). If this is done, Equation 3 becomes:

This means that, for an increase of the prestressing force by r(l+m),equilibrium between the forces is established if the spacing load isincreased at the same time by rm, or for the full cancellation of theinfluence of both chock spring and roll spring on the roll opening, theratio between the increase dP of the prestressing force and theincreases d8 of the spacing load must be:

which is a constant and the term mentioned before.

While it is preferred to make the ratio between the changes of theprestressing force and the spacing load equal to acceptable results maybe obtained when employing ratios which are slightly above or below thisvalue.

The method according to the invention therefore involves thesimultaneous measurement of the prestressing force and of the spacingload which is preferably done by separate instruments. The roll-spacingload may be measured by load cells which are placed in the path of theroll-spacing load, and the prestressing force may be measured by loadcells placed in the path of the prestressing force. In the case of theprestressing means being of the hydraulic type, the prestressing forcemay alternatively be measured by a pressure transducer connected to thefluid pressure chamber or chambers of the prestressing means, forinstance, to a cylinder in which a hydraulic ram is movable. Instead ofemploying load cells for the measurement of the spacing load, adistancemeasuring transducer may be employed which measures the changes,caused by that load, of the distance between the centers of the chocksof one roll and those of the chocks of another roll.

An embodiment of the invention will now be described by way of examplewith reference to the accompanying drawing, which shows diagrammaticallya rolling mill equipped with a control system according to the presentinvention.

The rolling mill shown is designed for the rolling of flat articles suchas sheet, strip or foil stock, and is of the four-high type. The millhas housings 10, of which only one is visible in the drawing. Eachhousing has a window 12 in which are accommodated an upper bearing chock14 and a lower bearing chock 16 for two back-up rolls 18. Upper andlower bearing chocks 22 and 24 for the working rolls 26 are mounted inrecesses of the chocks 14 and 16. Hydraulic cylinder-and-piston units 28and 30 are provided in the chocks 22 and 24 for supporting the latter inthe chocks 14 and 16.

Screw spindles 32 are mounted in the chocks 14 and abut against loadcells 34 received in recesses of the chocks 16. These spindles act asspacing means for the Working rolls 26, determining the size of theopening between them. The spindles 32 can be advanced or retracted 1relative to the checks 14 by being rotated in fixed nuts 33 by motors(not shown) whereby the opening between the rolls 26 is set andadjusted. The load cells 34 measure the spacing load S in the spindles32.

Each housing 10 is prestressed by hydraulic means comprising a cylinder36 and a ram 38 placed between the bottom of the housings 10 and thechocks 16. Pressure fluid in the cylinders 36 is supplied from a pump40, driven by a motor 42, the hydraulic circuit of the pump includingfurther a reservoir 44 and a servo control valve 46.

The prestressing force P in the housings 10 is measured by one or moreload cells 48 positioned either between the top of the housings and thechocks 14, as shown, or between the rams 38 and the bottom of thehousings. Alternatively, pressure transducers 50 connected to the spaceinside the cylinders 36 may be used.

The load cells 34, 48 and the pressure transducers 50 may be of anysuitable and known design and need not be described here. They registertheir measurements as electrical signals which are fed into a controlcircuit. This circuit includes a two-stage electrical comparator 52 towhich signals emanating from load cells 48 (or from the transducers 50)are fed through a variable resistor R1, and signals emanating from theload cells 34 through a variable resistor R2. In stage I of thecomparator, an intermediate signal s1 is produced by subtracting fromeach other simultaneously-arriving signals from the load cells 34 andfrom the load cells 48 (or from the transducer 50). This signal is fedinto stage II of the comparator 52, together with a constant referencesignal sr obtained from an adjustable potentiometer 54. This referencesignal represents the size of the roll opening which is required toproduce a rolled article of the required dimensions. By subtracting thereference signal sr from the intermediate signal sz', a controlling orerror signal e=sisr is obtained which is fed into a power amplifier 56.This amplifier feeds its output into the electro-hydraulic servo controlvalve 46 which operates in such a manner that an increase or decrease inthe output of the amplifier 56 results in a change of pressure in thepump 40 and therefore also in the cylinders 36 of the hydraulicprestressing means. The servo control valve 46 may be of any suitableand known type.

Prior to the starting of a rolling operation, the opening between theworking rolls 26 is set by means of the spindles 32 -for the desiredthickness of the rolled article. The cylinders 36 are filled withpressure fluid, producing a prestressing force P which exceeds themaximum separating force R to be expected under normal operatingconditions. The diflerence PR=S will thus always be positive.

Rolling is started with the automatic control system switched 05 by aswitch 66. The load meters 34 produce a signal representing the spacingload S in the spindles 32 and in the chocks 14 and 16, and the loadmeters 48 (or the pressure transducers 50) produce a signal representingthe prestressing force P in the housings 10. These signals aremultiplied by the resistors R1 and R2 or by potentiometers, the factorof multiplication being (k and (k +k respectively. The multipliedsignals are fed into stage I of the comparator 52 and produce therein anintermediate signal si=k P'(k -|-k )S. This signal is fed into stage IIof the comparator 52 and compared therein with the reference signal srfrom the adjustable potentiometer 54, whereby the controlling or errorsignal e=si-sr is obtained. The signal e can be read on indicator 70.The potentiometer is then so adjusted that 2 becomes zero, whereupon theautomatic control system is switched on at 66. From then onwards, anychanges in the separating force R due to changes in a physical propertyof the rolled material are reflected in corresponding changes of thesignal si and lead, owing to the operation of the automatic controlsystem, to changes in the prestressing force P by dP, and of the spacingload S by dS, so that the intermediate signal si becomes:

the plus and minus signs in the brackets corresponding to increases ordecreases of the separating force R. The error signal 2 which has beenreduced to zero will now be:

e'=sisr which can be resolved into:

' k dP(k1+k2)dS The electrical control system reduces the error signal2' again to zero, so that:

k dP: (10 k )dS dS m This has been shown above to be the condition foreliminating any changes in the size of the opening between the rolls 26,whether caused by chock spring or by roll spring. In this way, closetolerances of the rolled material can be maintained throughout a rollingoperation independently of any changes in the physical properties of therolled material.

The indicator 70 makes it possible to observe continuously the operationof the control system and in particular the reduction of the errors insignal e to zero. Thus, the correct operation of the system can beconstantly checked.

At least one of the resistors R1, R2 is preferably adjusted for initialcalibration and for taking into account any subsequent variations of theco-efficient m which may occur from time to time due to wear of therolls or other causes.

The invention is capable of variations in detail. It is, for instancepossible to multiply only one of the signals from which the intermediatesignal is produced. In this case, the multiplying factor is changed intohide a and the intermediate signal then becomes:

1. A method for controlling the operation of a prestressed rolling millhaving spacing means for setting the roll opening, said spacing meansbeing mounted in the bearing chocks of one roll and acting on thebearing chocks of the other roll, comprising the steps of measuringcontinuously during rolling, the load in said spacing means and theprestressing force, transforming these measurements into electricsignals, establishing a predetermined ratio between two signalsrepresenting simultaneous measurements of the prestressing force and thespacing load by multiplying at least one of these signals by a factorwhich includes co-efiicients representing both chock spring and rollspring and obtaining a signal for controlling the prestressing force bycomparing the two first-mentioned signals, after said multiplication,with each other, and with a reference signal representing the desiredthickness of the rolled material, and controlling the prestressing forcein accordance with the indication of said controlling signal, theprestressin-g force being so controlled that the influence of chockspring and roll spring on the roll opening is eliminated.

2. A method for operating a prestressed rolling mill according to claim1, in which the reference signal represents the roll opening whichproduces rolled material of a given thickness.

3. A method for operating a prestressed rolling mill according to claim1, in which the signal controlling the prestressing force is obtained intwo steps in such a manner that in the first step the signalsrepresenting the load in the spacing means and the prestressing forceare compared with each other, after multiplication, to form anintermediate signal, and that in the second step the intermediate signalis compared with the third or reference signal which represents thedesired thickness of the rolled article, to form the controlling signalfor the prestressing force.

4. A method for operating a prestressed rolling mill according to claim3, in which the intermediate signal is obtained by subtracting from eachother the signals rep resenting the spacing load and the prestressingforce respectively.

5. A method for operating a prestressed rolling mill according to claim3, in which the controlling signal for the prestressing force isobtained by subtracting from each other the intermediate and thereference signals.

6. A method for operating a prestressed rolling mill having rolls andchocks subject to elastic springing, two of said rolls comprising workrolls between which material to be rolled is passed, spacing means forsetting the opening between said work rolls, and means for applying aprestressing force, said method comp-rising the steps of measuring thespacing load and the prestressing force continuously during rolling,transforming said measurements into a pair of electrical signals,establishing a predetermined ratio between simultaneous values of saidpair of signals which ratio includes coefiicients representing chockspring and roll spring, comparing one signal of said pair with the othersignal of said pair after said ratio has been established, and using thecompared signal for controlling the prestressing force so that theinfluence of both chock spring and roll spring on the opening betweenthe Working rolls is canceled out.

7. A method for operating a prestressed rolling mill according to claim1, in which the predetermined ratio between the pair of signalsrepresenting simultaneous measurements of the prestressing force and ofthe spacing load is equal or substantially equal to l-l-m m wherein m isa constant and equal to k /k k and k being constant co-efiicients of thechock spring and roll spring respectively.

8. A rolling mill having housings with superposed rolls between which aseparating force is established during a rolling operation, bearingchocks supporting said rolls in said housings, means for setting thedistance between two super-posed chocks, prestressing means for applyinga prestressing force to said chocks and said housings, means formeasuring the load in said setting means, means for measuring theprestressing force and means for controlling the prestressing force sothat the difference between the prestressing force and theroll-separating force is kept constant during a rolling operation.

9. A rolling mill according to claim 8, in which said control meanscomprise means for developing a first signal representing the load insaid setting means, means for developing a second signal representingthe prestressing force, and means for comparing the first and secondsignals with each other and developing a third signal, said third signalbeing utilized for controlling the prestressing force so that thediiference between the prestressing force and the load in said settingmeans is kept constant during a rolling operation.

10. A rolling mill according to claim 9 comprising means for developinga fourth signal, said fourth signal representing the desired thicknessof the rolled material, and means for comparing said third and fourthsignals with each other so as to obtain an error signal for controllingthe prestressing force so that the difference between the prestressingforce and the load in said setting means is kept constant during arolling operation.

11. Rolling mill according to claim 8 in which the load in said settingmeans is measured by load cells and the prestressing force is measuredby load cells.

12. Rolling mill according to claim 11 in which the prestressing meansis of the hydraulic type and a pressure transducer is used to measurethe prestressing force.

13. Rolling mill according to claim 8- in which the load between saidbearing chocks is measured b-y distancemeasuring transducers.

14. Rolling mill according to claim 8 in which the prestressing means isof the hydraulic type and a servo-valve is used to control the hydraulicpressure in said prestressmg means.

References Cited UNITED STATES PATENTS 2,736,2l7 2/1956 Blain 72212,903,926 9/1959 Reichl 728 3,124,982 3/1964 Neumann 7221 3,159,063 12/1964 Fox 7221 3,247,697 4/ 1966 00220 72243 CHARLES W. LANHAM, PrimaryExaminer. A. RUDERMAN, Assistant Examiner.

